Interpretive Summary: Peanuts are commonly invaded by Aspergillus flavus and A. parasiticus, soil-inhabiting molds that contaminate peanuts by producing carcinogenic aflatoxins. Aflatoxin contamination of peanuts in the field can be reduced by 77 to 98% with biological control through the application of nontoxigenic strains of these species, which competitively exclude native aflatoxin-producing strains from invading peanuts. In this study, viable peanut seeds were artificially wounded and inoculated with field soil containing natural mold populations that were supplemented with different spore concentrations of nontoxigenic A. flavus and A. parasiticus. Reduction of aflatoxins in peanut seeds depended on both the density and the aflatoxin-producing potential of the native mold populations and on the applied strain used for biological control. This information will be useful for optimizing current biological control practices.

Technical Abstract:
Peanuts and other seed and grain crops are commonly contaminated with carcinogenic aflatoxins, secondary metabolites produced by Aspergillus flavus and A. parasiticus. Aflatoxin contamination of peanuts in the field can be reduced by 77 to 98% with biological control through the application of nontoxigenic strains of these species, which competitively exclude native aflatoxin-producing strains from developing peanuts. In this study, viable peanut seeds were artificially wounded and inoculated with field soil containing natural fungal populations that were supplemented with conidia of nontoxigenic A. flavus NRRL 21882 (niaD nitrate-nonutilizing mutant) and A. parasiticus NRRL 21369 (conidial color mutant). Increasing soil densities of applied nontoxigenic strains generally resulted in an increase in the incidence of seed colonization by applied nontoxigenic strains, a decrease in seed colonization by native A. flavus and A. parasiticus, and a decrease in aflatoxin concentration in seeds. Reduction of aflatoxins in peanut seeds depended on both the density and the aflatoxin-producing potential of native populations and on the fungal strain used for biological control. A wild-type strain of A. flavus NRRL 21882 and its niaD mutant were equally effective in reducing aflatoxins in peanuts, indicating that nitrate-nonutilizing mutants, which are easily monitored in the field, can be used for evaluating the efficacy of biocontrol strains